The need for more effective technologic solutions to the problems posed by both private and public passenger transport is now generally acknowledged. In particular, attention is increasingly focused on achieving a substantial reduction in consumption and a real reduction in pollutant and acoustic emissions, especially in urban areas. Both these problems are of universal concern, and there is not a car industry that is not currently engaged in researching and developing vehicles capable of providing at least partial solutions and reducing the effect these problems have on human living conditions.
This explains the increasingly widespread use of innovative vehicles equipped with highly efficient, low-pollution propulsion systems. Particularly interesting in this respect is the development of so-called “hybrid” vehicles known as HEVs (Hybrid Electric Vehicles). These are vehicles of various design integrating an electric generating system, a reversible storage system, and a drive system comprising an electric motor and a combustion engine, and are in turn classified, according to the degree of hybridization, as: “Mild HEV”, “Medium HEV” and “Full HEV”.
At present, the most commonly used vehicles are those in the minimum-hybrid or “Mild HEV” or “Mild Hybrid Electric Vehicle” class. In this case, to a vehicle of conventional design is added the possibility of employing an electric drive over very short distances or in particular short-term situations. It is important to note that even a minimum degree of hybridization has advantages in terms of reducing consumption and pollutant emissions, especially when driving in urban areas.
A conventional vehicle is equipped with one 12 volt lead-acid battery capable of supplying average power of 400-800 watts and peak power of 1500-3500 watts (only required when starting the engine). A “Mild HEV” vehicle, on the other hand, is equipped with a battery (or group of batteries) capable of supplying average power of 2500-3500 watts and peak power of 8000-11000 watts (only required when starting the engine).
In “Stop and Start” mode, the internal combustion engine is turned off when the vehicle is stationary or coming to a stop (typically for reasons of traffic, such as a red light or a give-way junction), and is started again when the driver presses the accelerator. During start-up, the electric starter motor consumes a considerable amount of electrical energy, thus greatly stressing and reducing the electric charge of the vehicle battery. In “Stop and Start” mode, therefore, intensive use is made of the vehicle battery, on account of the high frequency with which the internal combustion engine is started, particularly when driving in city traffic.
Regenerating mode also increases stress of the vehicle battery, on account of the battery having to absorb a large amount of electrical energy over a short period of time. In regenerating mode, deceleration of the vehicle is used to generate electrical energy, which is stored in the vehicle battery, thus recovering at least part of the kinetic energy which would otherwise be dissipated in the form of heat by a conventional brake system.
In a vehicle battery, controlling power and energy flow during sharp-discharge transients when starting up the internal combustion engine is a delicate business, particularly when the battery is called upon to power other electric devices at the same time. In fact, the reduction in the capacity of the battery caused by simultaneously-operating electric devices may seriously impair start-up of the internal combustion engine, on account of the high transient power required at this stage.
To tackle these problems, various solutions have been proposed, all mainly based on energy storage systems comprising a combination of batteries and banks of ultracapacitors. In known energy storage systems of this sort, energy flow is controlled by predetermined control logic requiring the manufacture and use of specific electronic control circuits, thus complicating the system and increasing both production and routine maintenance cost.